Compositions and methods for sintering powdered metal compacts using little or no hydrogen

ABSTRACT

Metal powder compositions for pressed powder metallurgy, and methods of forming metal parts using the metal powder compositions. The metal powder compositions include a metal powder and a lubricant system, which are pressed to form a green compact. When the green compact is heated during a delubing process, the lubricant system provides a reactive carbon coating on the surface of the metal powder, which upon subsequent sintering, reacts with metal oxides present on the surface of the metal powder to form gas that is easily removed from the system. Removing the metal oxides from the surface of the metal powder provides a more dense metal part, and allows for sintering of the green compact without the need for hydrogen gas, which would otherwise be needed to remove the metal oxides on the surface of the metal powder.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit from U.S. Provisional Patent Application Ser. No. 62/431,970 filed Dec. 9, 2016, which is incorporated herein by reference.

BACKGROUND OF INVENTION Field of Invention

The present invention relates to compositions and methods for sintering powdered metal compacts using little or no hydrogen gas in pressed powder metallurgy processes.

Brief Description of Related Art

In pressed powder metallurgy, a substantially dry metal powder composition is charged into a die cavity of a die press and compressed to form a green compact. Pressing causes the metal powder particles in the metal powder composition to mechanically interlock and form cold-weld bonds that are strong enough to allow the green compact to be handled and further processed. After pressing, the green compact is removed from the die cavity and sintered at a temperature that is below the melting point of the major metallic constituent of the metal powder composition, but sufficiently high enough to strengthen the bond between the metal powder particles, principally through solid-state diffusion.

In pressed powder metallurgy applications, a lubricant may be added to the dry metal powder composition before it is pressed to form the green compact. Lubricant helps the metal powder particles to move into all portions of the die cavity, allows for some particle-to-particle realignment during pressing, and serves as a release agent that facilitates removal of the green compact from the die cavity after pressing. The least amount of lubricant necessary to obtain good flow and release is usually used. This is because lubricants can also detrimentally impact green density and result in the evolution of undesirable effluents during delubing and the sintering operations. Lubricants can also contribute to low final density in metal parts, protracted furnace time necessary for removing the lubricants in a “delubing” operation, and the formation of cracks and blisters during sintering.

Conventionally, after pressing the lubricant is removed from the green compact in a delubing operation, which can involve gradually heating the green compact at a relatively low heating rate (e.g., about 15° F./min) until the lubricant melts, boils, and/or thermally decomposes (e.g. burns off) so that it is completely removed from the green compact. Delubing is typically accomplished during a preheating stage before sintering, or during an initial heating stage at the beginning of the sintering process. The lubricant is completely removed from the green compact at a temperature that is lower than the sintering temperature.

It is a common practice in the powdered metal processing of iron based powdered metal parts to sinter in a reducing atmosphere such as disassociated ammonia, endo gas, or a hydrogen/nitrogen mixture. This atmosphere helps to remove metal oxides from the surface of metal powder. Removal of the metal oxides from the surfaces of the metal powder allows the metal powder to more fully sinter to form a metal part having higher densities and/or better properties with density similar to the green density of the metal powder compact. However, all of these conventional systems have some safety, operational, and cost concerns. In recent years a hydrogen/nitrogen mixture has been mostly adopted due to convenience and in-plant part-to-part reproducibility. Hydrogen is generally believed to be necessary for effective sintering due to its ability to act as a reducing agent for the metal oxides that are commonly on the surfaces of metal powder. Iron powder for example typically contains 1,000 to 1,500 ppm or higher of oxides on the metal particle surface prior to sintering. Such oxides are believed to inhibit sintering together of the metal powder particles, and thus present a barrier to achieving an acceptable sintered density and acceptable resulting properties for the metal part.

Ideally, the sintered density of a final part would be 100% of the theoretical density of the metallic constituents of the metal powder composition used to form the part. However, the sintered density of parts formed from most conventional metal powder compositions does not approach 100% of theoretical density. Using conventional high carbon or low alloy steel metal powder compositions and pressed powder metallurgy methods, only a sintered density of about 93% to 94% of theoretical density can be achieved in one pressing and sintering. For stainless steels, sintered densities are typically less than 90% of theoretical density for conventional powder metallurgy compositions. Additional processing steps, such as forging and repressing are required to increase the density of the sintered metal part.

In recent years, powdered metal process plants have reduced the amount of hydrogen in the sintering atmosphere for reasons such as cost and safety. Typical sintering is presently conducted using an atmosphere consisting of between 5-25 volume percent hydrogen for iron sintered parts, with the balance being nitrogen. Some sintered parts use higher percentages of hydrogen including up to 100% for some alloys such as stainless steel. If the amount of hydrogen in the sintering atmosphere could be substantially reduced or eliminated, there would be significant cost savings (estimated to be up to 6 cents per pound of the final sintered product) and reduced safety hazards.

BRIEF SUMMARY OF THE INVENTION

In one aspect, a method a method of forming a metal part comprises providing a composition comprising metal powder and at least 0.5 wt % lubricant system. The lubricant system includes 10-30 wt % stearic acid, 0.1-5 wt % guanidine material, 0.1-0.8 wt % antioxidant, 5-15 wt % microcrystalline wax, 10-30 wt % polyethylene/polypropylene copolymer wax, and 40-60 wt % ethylene bis(stearamide). Pressure is applied to the composition to thereby form a green compact, and the green compact is delubed in a delubing atmosphere that contains less than 5 volume % hydrogen gas, and more preferably 0% hydrogen gas, to thereby form a metal part. The method can further include sintering the green compact in a sintering atmosphere that contains less than 5 volume % hydrogen gas, and more preferably 0% hydrogen gas.

In another aspect, a lubricant system comprises 10-30 wt % stearic acid, 0.1-5 wt % guanidine material, 0.1-0.8 wt % antioxidant, 5-15 wt % microcrystalline wax, 10-30 wt % polyethylene/polypropylene copolymer wax, and 40-60 wt % ethylene bis(stearamide). When the lubricant system is combined in an amount of at least 0.5 wt % with metal powder and compressed into a green compact, and the green compact is delubed and sintered in an atmosphere that contains less than 5 volume % hydrogen gas, and more preferably 0% hydrogen gas, to thereby form a metal part.

The foregoing and other features of the invention are hereinafter more fully described below, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a solid lubricant system for use in powder metallurgy. The lubricant system is solid at ambient conditions, but upon application of press pressure (forming pressure and stress) it transforms to a liquid phase. In addition to the lubricant system, the present invention also provides a composition further including metal powder, and a method of using the lubricant system and metal powder composition to form a metal part. The lubricant system of the present invention may also be used in connection with the pressing of ceramic powders.

The lubricant system provides lubrication during powder metallurgy processes, and results in good flow of the metal powder composition, ease of removal of the green part from the mold cavity, the formation of minimal effluents during heating, and can be used at low loading levels. Because less lubricant system is utilized, green density increases due to less volume of lubricant system and due to particle-to-particle rearrangement caused by slippage at low pressure. As green density improves, final part properties also improve (e.g., sintered density, strength, hardness, greater uniformity and fewer defects). By use of the lubricant system of the present invention, excellent green densities are achieved without the use of special equipment such as added heating devices as used in conventional hot pressing or the use of die wall lubricating systems.

Metal powder compositions according to the present invention comprise metal powder, e.g. one type or a blend/mixture of different types of metal particles, and a lubricant system, which can be include in an amount of at least 0.5 wt %, 0.5-3 wt %, 0.5-1 wt %, 0.5-0.6 wt %, or 0.5 wt % of the metal powder composition.

The metal powder and the lubricant system are mixed together to form a metal powder composition. The lubricant system is capable under pressure or heat, of transforming from a solid to a viscous liquid, and includes one or more organic compounds that upon delubing, are capable of depositing a reactive carbon residue on the metal powder. It is believed that the reactive carbon residue is effectively spread onto the surface of each iron particle since the lubricant transforms from a solid into a viscous liquid, thereby making a very efficient mechanism to coat the iron particles with reactive carbon residue.

Applicant theorizes that when the metal powder compositions according to the invention are delubed (which is also sometimes referred to in the art as “debound”) in nitrogen, one or more organic compounds present in the lubricant system react and/or thermally decompose to form highly reactive carbon-containing species that are deposited as a residue layer on the surface of the metal particles.

The carbon-containing species on the outer surface of the metal powder are thus available during subsequent sintering to react with the metal oxides on the outer surface of the metal particles to thereby form volatile compounds such as carbon monoxide and/or carbon dioxide. This mechanism removes metal oxides from the outer surface of the metal particles at temperatures well below where solid state diffusion and liquid phase formation occurs.

With the removal of metal oxides from the outer surface of the metal powder, the metal powder can therefore more fully sinter and achieve a metal part with better properties, and without the need for a hydrogen-containing sintering atmosphere.

Applicant previously disclosed compositions and methods for pressed powder metallurgy in U.S. Pat. No. 8,062,582, which is hereby incorporated by reference in its entirety. The compositions and methods disclosed therein sought to remove oxides from the outer surface of metal particles in situ in order to achieve near full density metal parts when powdered metal compacts formed of such particles were sintered. To achieve near full density upon sintering, such compositions and methods utilized relatively low loadings (0.1% to about 4% by volume) of an organics package, which was intended to leave only a small amount of a carbon residue on the outer surface of the metal particles subsequent to a delubing heating cycle.

Applicant has surprisingly discovered that one can modify the compositions and methods previously disclosed in U.S. Pat. No. 8,062,582 to produce sintered powdered metal compacts in nitrogen only that have properties equivalent or better than can be achieved in conventional sintering processes where hydrogen is present.

The metal powder included in the metal powder composition can comprise one or more populations of metal particles, including particles of a single metallic element (e.g., iron powder), pre-alloyed particles (e.g., low-alloy steel powders or stainless steels powders), agglomerations, blends, or mixtures of two or more populations of particles that are made from different metallic elements (e.g. a mixture of iron powder and nickel powder). Suitable metallic elements include, for example, iron, copper, chromium, aluminum, nickel, cobalt, manganese, niobium, titanium, molybdenum, tin and tungsten. It will be appreciated that metal powder compositions according to the invention can include other additive elements, such as bismuth, vanadium and manganese (typically in the form of manganese sulfide) for example, and other conventional additives.

The metal particles used in the metal powder compositions according to the invention tend to have outer surfaces that are oxidized, typically as a result of contact with oxygen in the atmosphere or with water vapor. Metal particles comprising iron, which are frequently used in pressed powder metallurgy to form steel parts, have surfaces that are oxidized in the form of iron oxide, which oxide is present typically at 1,000 to 1,500 ppm of the metal powder. Applicant believes that metal oxides on the surface of metal particles may interfere with solid-state diffusion bonding between such particles during sintering. The metal oxides on the surface of the metal particles may also inhibit the solid state diffusion and formation of liquid phase alloys, which can be used to solder, weld or otherwise bind the individual metal particles together.

In several embodiments, the lubricant system includes or consists of stearic acid, guanidine material, antioxidant, microcrystalline wax, polyethylene/polypropylene copolymer wax (“PE/PP wax”), and ethylene bis(stearamide). In one embodiment, the lubricant system is free of lauric acid.

The stearic acid may be included at 5-35 wt %, 10-30 wt % or 15-25 wt % of the lubricant system. A suitable stearic acid may be Emersol® 120, Emersol® 132 F, Emery® 400, Emery® 405, Emery® 410, Emery® 420, Emery® 422, Edenor® C1865 MY, Edenor® C1892 MY, and Emersol® 153 NF available from Emery Oleochemicals, Selangor, Malaysia. In one embodiment, a rubber grade stearic acid, such as Emery® 420 is used.

The guanidine material may be included at up to 5 wt %, 0.1-5 wt %, or 1-4 wt % of the lubricant system. In one embodiment, the guanidine material is a reaction product of guanidine and an acid selected from a fatty acid, an organic acid, or a stronger acid. The guanidine material is a reaction product which may be an amide or a hydrated salt. For example, according to the CRC Handbook of Chemistry and Physics, 74th Ed. guanidine acetate has the formula (H₂N)₂, C═NH.CH₃COOH, rather than an amide-type formula such as H₂N—C═NH(NH)COCH₃, as would be expected for an amide. This is due to the fact that guanidine is a very strong base, and is much more likely to simply extract a proton from a relatively weak organic acid, rather than react with the organic acid in a “standard” amidization reaction forming an amide with concomitant loss of H₂O. However, in some cases, the reaction of guanidine and the acid may yield an amide in the “standard” manner. For this reason, the guanidine material of the present invention will be referred to herein as the reaction product of guanidine and an acid. The term “reaction product of guanidine and an acid” includes both of the above-described forms of the product of a reaction between or mixture of guanidine and an acid, and mixtures of these forms or other possible forms.

The particular acid used to make the reaction product of guanidine and an acid is selected based upon obtaining desired effects when mixed with other compounds. In one embodiment, the guanidine material is guanidine stearate. In one embodiment, the guanidine material includes guanidine ethyl hexanoate. In other embodiments, the guanidine material may be the reaction product of guanidine and other acids.

According to the present invention, the guanidine material may include the reaction product of guanidine and other organic acids in the C₁₂ to C₂₂ range. Thus, for example the reaction product of guanidine and oleic acid (C₁₇H₃₃CO₂H) would be suitable. Other suitable acids include such saturated fatty acids as (common names in parentheses) dodecanoic (lauric) acid, tridecanoic (tridecylic) acid, tetradecanoic (myristic) acid, pentadecanoic (pentadecylic) acid, hexadecanoic (palmitic) acid, heptadecanoic (margaric) acid, octadecanoic (stearic) acid, eicosanoic (arachidic) acid, 3,7,11,15-tetramethylhexadecanoic (phytanic) acid, monounsaturated, diunsaturated, triunsaturated and tetraunsaturated analogs of the foregoing saturated fatty acids. Additional organic acids include acids such as ethylhexanoic acid (C₇H₁₅CO₂H), hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, and dodecanoic acid. Branched-chain carboxylic acids in the C₆ to C₁₂ range may also be used.

According to the present invention, the reaction product of guanidine and stronger acids such as sulfonates, phthalates, benzoates, phosphates and phenols may be used. For example, the reaction product of guanidine and an acid such as benzenesulfonic acid may be used. As an alternative, intermediate acids may be selected for reaction with guanidine. Alternatively, the guanidine material used in the lubricant system may be the reaction product of guanidine and a weaker acid such as benzoic acid.

In a one embodiment, the guanidine material comprises a mixture of guanidine stearate and guanidine ethyl hexanoate. A suitable guanidine material is APEX Special Purpose Additives—Surface Agent Mixture, available from Apex Advanced Technologies LLC, Cleveland, Ohio.

The antioxidant can be included at up to 1 wt %, 0.1-0.8 wt %, or 0.1-0.5 wt % of the of the lubricant system. Suitable antioxidants include, but are not limited to, tris (2,4-di-tert-butylphenyl) phosphite, his (2,4-dicumylphenyl) pentaerythritol diphosphite, bis (2,4-dicumylphenyl) pentaerythritol diphosphate, bis (2,4-dicumylphenyl) pentaerythritol diphosphite, distearyl pentaerythritol diphosphite, and combinations thereof. In one embodiment, the antioxidant is Doverphos® S-480, which is tris (2,4-di-tert-butylphenyl) phosphite available from Dover Chemical Corporation, of Dover, Ohio.

The microcrystalline wax can be included at 1-20 wt %, 3-18 wt %, or 5-15 wt % of the lubricant system. A suitable microcrystalline wax is a lamination grade microcrystalline wax available from Sovereign Chemicals & Cosmetics, of Maharashtra, India, which has a drop melting point of 70-80° C., a needle penetration at 25° C. of 25-45, an oil content of less than 2%, a viscosity at 98.9° C. of 15-20 CST.

The polyethylene/polypropylene copolymer wax (“PE/PP wax”) may be included at 5-35 wt %, 10-30 wt %, or 15-25 wt % of the lubricant system. A suitable PE/PP wax includes but is not limited to PE 520 available from Clariant International Ltd., of Charlotte, N.C.

The ethylene bis(stearamide) can be included at 35-65 wt %, 40-60 wt %, or 45-55 wt % of the lubricant system. A suitable ethylene bis(stearamide) is Struktol® TR EBS, or Struktol® TR EBS VG, available from Struktol Company of America, of Stow, Ohio.

The lubricant system thus preferably comprises 10-30 wt % stearic acid, 0.1-5 wt % guanidine material, 0.1-0.8 wt % antioxidant, 5-15 wt % microcrystalline wax, 10-30 wt % polyethylene/polypropylene copolymer wax, and 40-60 wt % ethylene bis(stearamide). In addition, the lubricant system or metal powder composition can optionally include various additives including a binder, a plasticizer, a degreasing promoting agent, a surfactant, etc. as desired. In another embodiment, the lubricant system consists essentially of 10-30 wt % stearic acid, 0.1-5 wt % guanidine material, 0.1-0.8 wt % antioxidant, 5-15 wt % microcrystalline wax, 10-30 wt % polyethylene/polypropylene copolymer wax, and 40-60 wt % ethylene bis(stearamide).

In a method of forming a metal part by powder metallurgy, a metal powder composition as disclosed herein is provided. The composition can include metal powder, and a lubricant system including stearic acid, guanidine material, antioxidant, microcrystalline was, PE/PP wax, and ethylene bis(stearamide) in the amounts as discussed herein. The method includes one or more of arranging the composition in a die cavity, applying pressure to the composition to thereby form a green compact, removing the green compact from the die cavity, delubing the green compact, and sintering the green compact to thereby form a metal part.

In one embodiment, the metal powder composition is arranged in a die cavity and pressed to form a green powder metal compact. During pressing of the composition in the die cavity, the lubricant system transforms from a solid to a viscous liquid and then flows around each metal particle and deposits onto an outer surface of the metal powder particles, and also flows to the walls of the die cavity to aid in release of the green compact.

In an embodiment, the green compact is delubed in a substantially non-hydrogen-containing atmosphere such as nitrogen. The one or more organic compounds that are part of the lubricant system, are capable of being reduced during the delubing step to form reactive carbon-containing species on the surface of the metal powder particles. One or more of the organic compounds present in the lubricant system decompose by being “carburized” during delubing. The term “carburize” as used in this application means that one or more of the organic compounds present in the lubricant system react or otherwise thermally decompose to form reactive carbon-containing species that are deposited as solids in the form of a layer or coating on the outer surface of the metal particles during delubing. In one embodiment, the microcrystalline wax is the component in the lubricant system that provides the most reactive carbon residue on the metal powder. This reactive carbon coating substantially reduces, if not eliminates, the need for using a hydrogen gas atmosphere during sintering. Optimally, the layer of reactive carbon-containing species is present in an amount sufficient to reduce oxides on the surface of the metal particles as the temperature during the sintering process rises to final sintering temperature, but without substantially imparting carbon into the sintered metal part. Ideally, the one or more organic compounds present in the lubricant system are selected such that the carbon in the carbon-containing species deposited on the surface of the particles is present at a molar weight ratio so as to be capable of removing oxides on the metal surface (e.g., the carbon to oxygen molar weight ratio is 2.66 to 1 for carbon dioxide, and 1.33 to 1 for carbon monoxide) during sintering. The reactive carbon-containing species have the ability to react with metal oxides to form carbon monoxide or carbon dioxide, and without diffusing carbon into the metal part in significant amounts.

One or more of the other organic compounds present in the lubricant system may “vaporize” during delubing. The term “vaporize” as used in this application means that one or more of the other organic compounds present in the lubricant system react or otherwise thermally decompose to form volatile gases, which are removed from the green compact during delubing.

Delubing and sintering can be performed in an atmosphere that contains less than 5 volume % hydrogen gas. In one embodiment, the delubing and/or sintering atmosphere contains no intentionally added hydrogen gas. In another embodiment, the delubing and/or sintering atmosphere is free of hydrogen gas. In one aspect, the delubing and/or sintering atmosphere include or consists of nitrogen gas. Other inert gases can be used, such as argon. During delubing in a nitrogen atmosphere, certain organic compounds included in the lubricant system react or otherwise thermally decompose to form a highly reactive carbon-containing species that are deposited as solid residue in the form of a layer or coating on the outer surface of the metal particles.

Sintering is preferably conducted in an inert atmosphere, such as nitrogen, because an inert atmosphere allows the reactive carbon residue and the metal oxide on the surfaces of the metal particles to react with each other. A hydrogen atmosphere could cleave the organics and/or interfere with the oxygen-scavenging/carbon residue producing reactions. Delubing and/or sintering in a vacuum would promote vaporization of the organics, which again would interfere with the desired reactions.

Another mechanism for removing the metal oxides may occur when the lubricant system comprises an organic acid and/or an organic compound having acid-functional groups. The acid may be available to react with metal oxides on the outer surface of the metal particles to form a metal salt residue, which can be reduced to elemental metal during sintering.

Both mechanisms may remove metal oxides from the outer surface of the metal particles at temperatures well below where solid state diffusion and liquid phase formation occurs. This can result in a complete or partial removal of oxides and significantly “cleaner” outer surfaces of the metal powder (i.e. less oxides present) that make the powder more susceptible to solid state diffusion and liquid phase bonding during sintering. In other words, the absence of an oxide layer, which is stripped during the delubing step, yields metal particles having very “clean” (i.e., oxide-free or having very low amounts of oxide residues) surfaces, which are capable of bonding and fusing together without the need for liquid phase forming materials or precursors thereof.

During a subsequent sintering step, the reactive carbon can react with metal oxides already present on outer surface of the metal particles to form carbon dioxide and/or carbon monoxide, which are removed as gases prior to solid state diffusion and liquid phase bonding.

The present subject matter metal powder compositions including the instant lubricant system provides benefits such as lower tons per square inch (TSI) needed to make a green compact, lower ejection force required to remove the green compact from a die cavity, reduced amount of lubricant system needed in the metal powder composition, improved green strength of the green compact, lower dimensional change in the sintered metal part, and allows for sintering without hydrogen gas, or at least allows for significantly reduced levels of hydrogen gas in the sintering atmosphere.

EXAMPLES

Several evaluations were conducted in order to access the benefits of the present subject matter as follows. Table 1 shows data for evaluations of inventive Example 1 in relation to Comparative Examples 1 and 2.

TABLE 1 Example Inventive Comparative Comparative Example 1 Example 1 Example 2 Metal powder FC 0208 FC 0208 FC 0208 Lubricant 0.5 wt % inventive 0.75 wt % Acrawax 0.75 wt % System lubricant system Caplube L Required TSI 36 TSI 50TSI 45TSI Green Density 7.02 7.02 7.01 Peak 2023 1898 1742 Slide 1483 1725 1300 G.S. 1758 PSI 1512 PSI 1599 PSI Dimensional 0.21% 0.25% 0.26% Change

As seen in Table 1, Inventive Example 1 included metal powder FC0208, which is a copper/iron metal powder, and an inventive lubricant system including 10-30 wt % stearic acid, 0.1-5 wt % guanidine material, 0.1-0.8 wt % antioxidant, 5-15 wt % microcrystalline wax, 10-30 wt % polyethylene/polypropylene copolymer wax, and 40-60 wt % ethylene bis(stearamide). Comparative Examples 1 and 2 included the same metal powder as Inventive Example 1, but Comparative Example 1 included Acrawax® C as a lubricant, which is an N,N′ ethylene bis(stearamide), available from Lonza, Basel, Switzerland; and Comparative Example 2 included Caplube L as a lubricant, which is available from H.L. Blachford, Montreal Canada. All three examples were similarly pressed, delubed, and sintered to form metal parts. As can be seen, Inventive Example 1 required less amount of a lubricant system, but produced a sintered metal part that had a comparable green density, increased peak value, comparable slide value, increased green strength value, and less dimensional change than Comparative Examples 1 and 2.

Table 2 shows data for evaluations of Inventive Examples 2-4 including different metal powders, which were sintered in a nitrogen-only atmosphere (indicated as “N₂”), and in an atmosphere including nitrogen and 10 volume % hydrogen atmosphere (indicated as “H₂”).

TABLE 2 Inventive Inventive Inventive Inventive Inventive Inventive Example Example Example Example Example Example Example 2 - H₂ 2 - N₂ 3 - H₂ 3 - N₂ 4 - H₂ 4 - H₂ Metal powder FC0208 FC0208 FY-4500 FY-4500 F-0008 F-0008 Inventive lubricant system 0.5 wt % 0 .5 wt % 0.5 wt % 0.5 wt % 0.5 wt % 0.5 wt % Green Density 7.0 7.0 7.29 7.29 6.8 6.8 Sintered Density 6.88 6.90 7.32 7.32 6.98 6.98 Hardness 86 86 69 70 69 69 TRS 158,426 158,570 118,001 132,051 110,847 109,192 Surface Appearance Good Good Good Good Good Good

Metal powder FY-4500 is an iron phosphorous steel with 0.45% phosphorous and the remainder iron. Metal powder F-0008 is an iron carbon steel with 0.6-0.9% carbon and the remainder iron.

Inventive Examples 2-4 were each sintered for 20 minutes at 2050° F. in a commercial furnace that is used for routine production. Inventive Example 2 had a Hall Flow 32.4 seconds, an apparent density of 2.98 g/cc, a carbon content of 0.84 before sintering and 0.804 after sintering in nitrogen only. Inventive Example 3 had a Hall Flow of 29.8 sec, an apparent density of 2.98 g/cc, a carbon content of zero added carbon before sintering and 0.038 after sintering in nitrogen only. Inventive Example 4 had a Hall Flow of 32.1 sec, an apparent density of 3.14 g/cc, a carbon content of 0.79 before sintering and 0.753 after sintering in nitrogen only. As can be seen in Table 2, the properties when sintering in a nitrogen-only atmosphere were comparable to those when sintering in an atmosphere including nitrogen gas and 10 volume % hydrogen gas.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and illustrative examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

1. A method of forming a metal part comprising: providing a composition comprising metal powder and at least 0.5 wt % of a lubricant system, the lubricant system including 10-30 wt % stearic acid, 0.1-5 wt % guanidine material, 0.1-0.8 wt % antioxidant, 5-15 wt % microcrystalline wax, 10-30 wt % polyethylene/polypropylene copolymer wax, and 40-60 wt % ethylene bis(stearamide); applying pressure to the composition to thereby form a green compact; delubing the green compact in a delubing atmosphere that contains less than 5 volume % hydrogen gas; and sintering the delubed green compact to thereby form the metal part.
 2. The method according to claim 1, wherein the delubing atmosphere is free of intentionally added hydrogen gas.
 3. The method according to claim 1, wherein the delubing atmosphere consists essentially of nitrogen gas.
 4. The method according to claim 1, wherein the composition is free of lauric acid.
 5. The method according to claim 1, further comprising arranging the composition in a die cavity before applying pressure to the composition.
 6. The method according to claim 5, further comprising removing the green compact from the die cavity before delubing and sintering the green compact.
 7. The method according to claim 1, wherein metal oxides are present on outer surfaces of the metal powder prior to delubing.
 8. The method according to claim 7, wherein during delubing, the lubricant system decomposes and leaves a carbon residue on the outer surfaces of the metal powder.
 9. The method according to claim 8, wherein the green compact is sintered in a sintering atmosphere that contains less than 5 volume % hydrogen gas.
 10. The method according to claim 9, wherein during sintering, the metal oxides present on the outer surfaces of the metal powder are removed in situ in a reaction with the carbon residue.
 11. The method according to claim 9, wherein the sintering atmosphere is free of intentionally added hydrogen gas.
 12. The method according to claim 11, wherein the delubing atmosphere consists essentially of nitrogen gas.
 13. The method according to claim 1, wherein the composition includes 0.5-0.6 wt % of the lubricant system.
 14. The method according to claim 1, wherein the guanidine material is a mixture of guanidine stearate and guanidine ethyl hexanoate.
 15. The method according to claim 1, wherein the antioxidant is tris (2,4-di-tert-butylphenyl) phosphite.
 16. The method according to claim 1, wherein the metal powder includes iron.
 17. A method of forming a metal part comprising steps of: providing a composition comprising metal powder and at least 0.5 wt % of a lubricant system; applying pressure to the composition to thereby form a green compact; delubing the green compact in a delubing atmosphere that contains less than 5 volume % hydrogen gas; and sintering the delubed green compact to thereby form the metal part; wherein the lubricant system transforms from a solid phase material to a viscous liquid phase material during the applying pressure step and thereby spreads onto outer surfaces of the metal powder, wherein after the delubing step a carbon residue is left on the outer surfaces of the metal powder due to decomposition of the lubricant system, and wherein during sintering, the metal oxides present on the outer surfaces of the metal powder are removed in situ in a reaction with the carbon residue.
 18. A lubricant system comprising: 10-30 wt % stearic acid, 0.1-5 wt % guanidine material, 0.1-0.8 wt % antioxidant, 5-15 wt % microcrystalline wax, 10-30 wt % polyethylene/polypropylene copolymer wax, and 40-60 wt % ethylene bis(stearamide), wherein when the lubricant system is combined in an amount of at least 0.5 wt % with metal powder and compressed into a green compact, and the green compact is delubed and sintered in an atmosphere that contains less than 5 volume % hydrogen gas to thereby form a metal part.
 19. The lubricant system according to claim 18, wherein the composition is free of lauric acid.
 20. The lubricant system according to claim 18, wherein particles of the metal powder have outer surfaces comprising metal oxide. 